To create water, you have to do more than simply mix hydrogen and oxygen molecules. You also have to introduce energy into the equation so that the atoms rearrange to fit together. The process is dangerous, both requiring and releasing substantial amounts of energy, which helps explain why we aren't able to safely produce water on Earth yet. And we still aren't even entirely sure how Earth came to be swimming so greatly in water.
For such a crucial compound, with its many uses and the fact that we would not have life without it, water has fairly simple chemical properties. As alluded to earlier, a water molecule contains one oxygen atom and two hydrogen atoms. These atoms are bound together by shared electrons. The natural water molecule has a 'V' shape; in electrical terms, it is polar, meaning that one side of the molecule carries a negative charge (the oxygen atom) and the other side carries a positive charge (the hydrogen atoms). In this way, a water molecule is somewhat like a magnet. Because of the natural polarity, water molecules attract one another and stick together, forming hydrogen bonds. When frozen, the joined water molecules take on crystalline structures to form ice. Due to its unique properties, ice floats on liquid water, which is unusual, because most compounds are denser as solids than they are as liquids.
Substances that dissolve in water are called hydrophilic substances: they "like" water, and they are able to mix with it easily. Many materials are hydrophilic - - after all, water is known as the "universal solvent." But there are substances that do not mix well with water. These are known as "hydrophobic" substances, and although the name implies that they're afraid of water, it really means that they don't break down within it. Sugar and salt, for example, are hydrophilic, but oil is hydrophobic.
Water itself is rarely pure, because many substances dissolve in it so easily. When water contains a certain array of dissolved minerals, it is considered "hard" water. Magnesium and calcium are common among the minerals in water, as are some metals. Hard water isn't especially bad for you (if it's coming out of your taps, you can assume that it's been treated and is considered safe, drinkable water), but you may find that soaps don't lather as well in hard water as they would in soft water. Hard water can also affect pipes and fixtures, leaving lime scale deposits and other calling cards.
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